Standardize prime plugin in charmcraft.yaml (#397)

I'd like to standardize the approach across data platform repos so that
future automated tooling that may need to use this can be shared across
all our repos

PRs in other repos:
canonical/mysql-operator#496

canonical/mysql-router-k8s-operator#298 (comment)
canonical/mongodb-operator#455

charmcraft.yaml

@@ -11,7 +11,8 @@ bases:
         channel: "22.04"
 parts:
   files:
-    plugin: nil
+    plugin: dump
+    source: .
     prime:
       - charm_version
       - workload_version